Glutamate is the major excitatory neurotransmitter regulating normal physiologic activity in the brain. Excessive glutamate release leads to excitotoxicity, which plays a prominent role in many disorders of the nervous system, including trauma and ischemic brain injury. Further, dysfunctional glutamate neurotransmission contributes to seizures and neurodegenerative disorders. Glutamate excitotoxicity is mediated largely through influx of calcium through the N-methyl-D-aspartate (NMDA) receptor leading to activation of PAR polymerases (PARP) and generation of Poly(ADP-ribose) (PAR) polymer, a newly described death signal that kills cells through apoptosis inducing factor (AIF). Genetic deletion of PARP-1 or drug inhibition results in profound neuroprotection. This form of cell death has recently been designated parthanatos to distinguish it from apoptosis, autophagy and necrosis. Parthanatos is prominently implicated in models of diabetes, inflammation, MPTP toxicity, myocardial infarction and cerebral ischemia.
Under physiologic conditions, normal bursts of excitatory activity result in synaptic transmission and the expression of molecular substrates of long-term plasticity, growth and survival. The activation of NMDA receptors in glutamatergic neurons plays a prominent role in inducing these long-lasting synaptic changes through multiple downstream signaling molecules and changes in gene expression. NMDA receptor stimulation may also be important for long-term changes that lead to neuronal survival.
Protein ubiquitination is a major regulatory process that controls a variety of cellular functions. Covalent modifications of proteins by ubiquitin can either mediate protein interactions or target the proteins for degradation depending on the nature of the ubiquitin modification. Conjugation of ubiquitin to a substrate uses a complex of proteins composed of an E1 ubiquitin activating enzyme, an E2 ubiquitin conjugating enzyme and an E3 ubiquitin ligase. E3 ligases are involved in substrate recognition and transfer of the ubiquitin molecule to the lysine residue on the substrate. Ubiquitin conjugation is activated and regulated by a few cellular signals. Phosphorylation is a well-studied intracellular signaling motif that marks proteins for the ubiquitination machinery. SUMOylation of proteins also appears to be a signal for ubiquitin modification and proteasomal modification. Other mechanisms of substrate recognition are not as well characterized.
PAR modification (PARsylation) of proteins is as an important cellular signaling mechanism. Proteins are PARsylated by PARPs. PARsylation regulates the function of a variety of nuclear proteins. Proteins can be covalently modified by PARP with PAR of different size and complexity, but proteins can also bind PAR non-covalently at specific PAR binding sites to regulate cellular signaling. For instance, PAR can act as a cytosolic signaling molecule during parthanatos.
RNF146 is a RING (really interesting new gene) finger protein that contains a WWE domain. This protein has been identified as a NMDA glutamate-receptor inducible gene in a genetic screen as clone PLING932.